Electrostatic printing material and method of its preparation

ABSTRACT

AN ELECTROSTATIC PRINTING MATERIAL HAVING IMPROVED PRINT DENSITY AND A METHOD OF ITS PREPARATION WHERE THE ELECTROSTATIC PRINTING MATERIAL COMPRISES AN ELECTRICALLY CONDUCTIVE SUPPORT LAYER AND A SECOND LAYER, THE SAID SECOND LAYER COMPRISING A POLYMERIC BINDER CONTAINING A DISPERSION OF A PHOTOCONDUCTIVE MATERIAL, WHERE THE POLYMERIC BINDER IS A POLYMER COMPRISING FROM ABOUT 50 TO ABOUT 65 PERCENT BY WEIGHT UNITS DERIVED FROM ETHYL ACRYLATE, FROM ABOUT 3 TO ABOUT 10 PERCENT BY WEIGHT UNITS DERIVED FROM AT LEAST ONE COMPOUND SELECTED FROM THE GROUP CONSISTING OF HYDROXYETHYL METHACRYLATE AND HYDROXYPROPYL METHACRYLATE, FROM ABOUT 30 TO ABOUT 45 PERCENT BY WEIGHT UNITS DERIVED FROM STYRENE AND UP TO ABOUT 10 PERCENT BY WEIGHT UNITS DERIVED FROM MONOCHLOROSTYRENE.

cilities to prevent undue exposure to light.

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a -.Int.-Cl.'G03g5/ US. Cl. 96--71.8 r 9 Claims ABSTRACT OF DISCLOSURE An electrostatic printing material having improved print density and a method of its preparation where the electrostatic printing material comprises an electrically conductive support layer and a second layer, the said second layer comprising a polymeric binder containinga dispersion of a photoconductive material, where the polymeric binder is a polymer comprising from about 50 to about 65 percent by Weight units derived from ethyl acrylate, from about 3 to about 10 percent by weight units derived from atleast one compound selected from the group consisting of hydroxyethyl methacrylate and hydroxypropyl methacrylate, from about 30 to about 45 percent by weight units derived from styrene and up to about 10 percent by weight unitsderi'ved from monochlorostyrene.

This invention relates to improved electrostatic printing materials and to a method of their preparation.

In electrostatic printing; an electrostatic printing material is generally prepared'by depositing or dispersing a In practicing the direct electrostatic process, the elec- 3,554,747 Patented Jan. I 12, 197 1 the composition and the improved electrostatic printing paper.

In accordance with this invention it has been found unexpectedly that an electrostatic printing material suitable for use in an electrostatic reproduction having improved print density comprises an electrically conductive support layer and a second layer, the said second layer comprising a polymeric binder containing a dispersion of a photoconductive material, where the polymeric binder is a polymer comprising from about 50 to about 65 percent by weight units derived from ethyl acrylate, from about 3 to about 10 percent by weight units derived from at least one compound selected from thegroup consisting of hydroxyethyl methacrylate and hydroxypropy-l methacrylate, from about 30 to about-' percent by weight units derived from styrene and up to about 10 percent by weight units derived from monochlorostyrene. The polymeric binder preferably has an intrinsic viscosity of about 0.2 to about 0.5 when determined in chloroform as a solvent at 30 C.

The electrostatic printing material can be prepared by a method which comprises coating an electrically conductive support with a solution of the polymeric binder containing the photoconductive material and drying the mixture of the polymeric binder and photoconductive material.

The polymeric binder of this invention is preferably prepared by solution polymerizing a monomer mixture which comprises from about to about 65 percent by weight ethyl acrylate from about 3 to about 10 percent by weight of at least one compound selected from the group consisting of hydroxyethyl methacrylate and hydroxypropyl methacrylate, from about 30 to about 45 percent by weight styrene and up to about 10 percent by weight units derived from monochlorostyrene in the presence of a suitable initiator in a solvent which will trostatic printing material is exposed to a suitable corona charged in proportion to the intensity of the light rays striking the material. The image is developed utilizing a liquid or powdered developer to form a visible reproduc-' tion of the image.

after. Electrostatic copying papers generally available to the art are'sens'itive'to light rays andrequire storage in complete darkness for a' period of time before being exposed to a corona discharge in order to" produce images having high contrast. Such dark-resting periods generally range from a few minutes to an hour or more. .These papers require care in handling and adequate storage fa- Print density is strongly related to the amount of electrostatic charge that can be deposited upon andassumed by the photoconductive material with a given corona discharge. Therefore, a high electrostatic charge on the photoconductive'material is generally desired for a high print density and electrically insulative binders which permit the photoconductive material to assume a high electrostatic charge. e

It is thereforean object of this invention to provide an electrostatic printing'paper having improved print density and dark-resting properties. It is another object to provide a new composition comprising a photoconductive material deposited in an electrically insulative composi.

tion. A further object isto providea method ofmaking yield a polymer having an intrinsic viscosity of from about 0.2 to about 0.5 when the conversion of the monomers to the polymer is at least about 97 percent complete. Although this specification teaches that it is preferred that a solvent is selected which will yield a polymer having an intrinsic viscosity of from about 0.2 to about 0.5 at about at least a 97 percent conversion, this is a characterization of the solvent and it is not required that the monomers be at least 97 percent converted to the polymer. If desired, small amounts of other monomers containing carboxylic groups can be mixed with the monomer mixture Electrostatic copying papers-producing images-having; to enhance the ability of the polymeric binder to adhere high contrast and high print density are continually sought to an electrically conductive substrate. An example of such a material is methacrylic acid when used in amounts of from about 0.1 to about 1.0 part by weight per 100 parts of the polymeric binder.

In the practice of this invention, it has been found that the polymeric binder when prepared by solution polymerizing the monomers has a unique utility for providing electrostatic printing papers having improved print density which do not require a dark-resting time. t 7 Various solvents can be used in the preparation of th solution polymer used for the polymeric binder. Representative of the various solvents are solvent mixtures comprising from about 50 to about percent by weight aromatic hydrocarbons such as xylene, toluene and benzene, and from about 50 to about 10 percent by weight aliphatic hydrocarbons, aliphatic alcohols and alcohol ethers. Representative of various aliphatic hydrocarbons are the pentanes, the hexanes, the heptanes, and the octanes. Representative of the various aliphatic alcohols are ethyl alcohol, propyl alcohol, and butyl alcohols. Representative of alcohol ethers is the mono methyl ether of ethylene glycol. The solution polymerization reaction can be promoted by various free radical initiators well known to those 3 skilled in the art for promoting free radical solution polymerizations. Representative examples of the various free radical initiators are benzoyl peroxide, ditertiary butyl peroxide, lauroyl peroxide, capryloyl peroxide, and azo-bis-isobutyronitrile.

The amount of free radical initiator employed in the polymerizations may be varied over wide concentrations. Of course, a catalytic amount of the initiator must 'be employed to cause polymerization of the monomers. The optimum amount of initiator depends on a number of factors such as temperature, reactants used, purity of reactants, reaction times desired and the like. Those skilled in the art will readily determine the optimum catalytic ranges.

A wide range of temperatures can be used for the polymerization such as from about 70 C. to about 120 C. although higher or lower temperatures can be used so long as a polymer having an intrinsic viscosity of from about 0.2 to about 0.5 can be produced when the conversion of the monomer is at least about 97 percent. Usually the time for the polymerization reaction to reach at least about a 97 percent conversion can be various times depending upon the initiator, solvent, and reaction temperature such as from about 5 to about 20 hours. The polymerization reaction can be conducted at atmospheric pressure or, if desired, it can be carried out at sub-atmospheric pressure or super-atmospheric pressure.

In the preparation of the electrostatic printing material of this invention, various photoconductive materials can be used with the polymeric binder to form the photoconductive layer. Representative of such materials are selenium, zinc oxide, cadmium sulfide, cadmium telluride, anthracene, sulfur, and mixtures of these compounds. It is usually preferable to use particulate zinc oxide. General- 4 at least 10- ohm and usually at least 10- ohmcm.-

The following examples illustrate the invention. In the examples, parts and percentages are by weight unless otherwise indicated.

EXAMPLES 1-10 A series of electrostatic printing papers were prepared with the polymeric binder of this invention prepared by solution polymerization and are herein identified for the purpose of this specification as Examples 1 through 10.

The polymeric binders for the electrostatic printing papers were prepared by the following method:

A solvent consisting of a mixture of xylene and mono methyl ether of ethylene glycol or butanol was charged to a suitable reactor equipped with a stirrer and refiux condenser. The solvent mixture was heated to cause the solvent mixture to reflux in the reflux condenser. To the reactor was then charged styrene, mono chloro styrene, if used, ethyl acrylate, and hydroxyethyl methacrylate or bydroxypropyl methacrylate in various amounts as shown in Table 1 along with small amounts of free radical reaction initiators such as ditertiary butyl peroxide at the higher polymerization temperatures and benzoyl peroxide at the lower polymerization temperatures. Small amounts of methacrylic acid were included to enhance the ability of the polymer to adhere to an electrically conductive substrate. All of the amounts of the reactants in Table 2 are in parts by weight. The monomers were polymerized at the temperature ranges shown in Table 1 for about 10 to 18 hours. The polymeric binders prepared as Examples 1-10 in Table 2 correspond to Examples l-10 of electrostatic printing papers to be prepared from these polymeric binders.

TABLE I.PREPARATION OF POLYMERIC BINDER Solvent system Mono methyl Monomers ether of Reaction ethylene Ethyl Methtemp., C. Xylene glycol Styrene acrylate acrylate Initiator Example:

1-. 130 50 30 l 10 Z 1. 0 -80 20 30 60 l 10 3 l. 5 75-80 80 4 20 30 60 l 10 3 1. 5 76-80 10 30 60 l 10 3 1. 5 75-80 90 10 5 30 64 1 6 3 1. 5 120 50 50 60 30 20 2 1. 0 120 50 50 30 70 0 2 1. O 120 60 50 30 64 ll 6 120 50 50 30 62 0 8 1 1.0 120 50 50 30 60 l 10 2 1. 0

1 Hydroxyethyl methacrylate containing 2.3 percent by weight of methacrylate acid. 2 Ditertiary butyl peroxide.

3 Benzoyl peroxide.

4 Butanol was used instead of the mono methyl ether of ethylene glycol.

6 0.1 part methacrylic acid added.

6 Hydroxypropyl methacrylate containing 2.6 percent by weight of methacrylic acid.

ly the photoconductive materials are used in particle sizes of from about 0.2 to about 2 microns. The amount of the photoconductive material used in the photoconductive layer is generally about 10 to about 1,200 percent by weight based on the solution polymer used in this layer and usually from about 500 to about 700 percent based on the solution polymer is desirable.

In the practice of this invention, the photoconductive material is deposited, and generally it is dispersed in particulate form in a solution of the polymeric binder. It is usually preferred that the insulative film suitable for electrostatic reproduction is prepared by dispersing particles of the photoconductive material in a solution of the said polymeric binder and then allowing the solvent to evaporate from the mixture. Generally, the mixture is coated onto the electrically conductive support following which the solvent is evaporated.

Various electrically conductive materials are generally suitable for use as a base in the electrostatic copying material and are generally known to those skilled in the art. The materials used usually have a specific conductivity of The solutions of polymeric binders were further diluted with toluene to form coating solutions at about 25 C. according to the following recipe:

Material: Parts by weight Solution of polymeric binder (50% solids by weight) 42 Particulate zinc oxide 120 'Dye (33',55'-tetrabromophenol-sulfone phthalein as a 0.005 gram/milliliter of methanol solution) 0.006 Toluene hered to thesecond layer'ofelectrically conductive paper; Theelectrostatic-printing papers were then inserted into a Sm ith Corona Marchant No. 33 commercial elec- 6 polymeric binder and the'unitsderived from styrene and ethyl acrylate in the polymeric binder were adjusted to 37 parts and 52 parts respectively.

TABLE TIL-COMPARATIVE EVALUATION OF PREPARED ELECTRO- STATIC PRINTING PAPERS Nodarkresta Print density Maxnn um charge (volts Charg 4. v 1' 1' j Dark decay (volts/see.) Light decay (v0lts/sec.) 94.6 Background N After 60 hours dark rest:

Print density Go Maximum charge (volts) 310 Charge rate (volts/see.) 'Dark decay (volts/see.)

Paper (A) (irom SCM paper Example Paper (B) Paper (0) 6.7. o No Yes Yes (slight).

O Light decay (volts/sec)... 91.2.. 60.7. Background No N0 Yes Yes (slight). Adhesion Fair Very good--. Excellent--- Excellent. Solvent resistance (36KB) do Excellent do Do. Zinc oxide tolerance do do Do.

trost'at ic copying machine in which they' were electrostatically charged by a corona discharge source, exposed to reflected imageo f" printed matter which was exposed to a light source,"and developed in the copying machine.

Each of the prepared electrostatic printing papers was evaluated as shown in Table II for print density, ad-

hesion of the polymeric binder to the electrically conductive paper, and whether dark resting is requ1red. h

TABLE IL-EVALUATION OF ELECTROSTATIC PRINTING PAPERS Comparative results of several prepared electrostatic printing papers and a commercial paper are shown in Table III both with respect to no dark resting of the papers and with respect to a 60 hour dark resting period before exposing the papers to the corona discharge as to print density, maximum charge, charge rate, dark decay, residual charge after 4 seconds, and whether background is present.

The electrostatic printing papers compared in Table III are the paper (A), a commercial electrostatic printing paper obtained as Microstatic Copy Paper Type 450 from the SCM Corporation, the paper (B), prepared according to Example 5, a paper (C), prepared according to Example 5 except that a temperature of about 120 C. was used to prepare the polymeric binder and the units derived from styrene and ethyl acrylate in the binder were adjusted to 42 and 52 parts respectively; and a paper (D), prepared according to the method of Example 5 except that five parts of monochloro styrene were polymerized with the monomers to prepare the It is known that the photoconductive materials used in this invention generally have an intrinsic sensitivity that lies substantially in the ultra-violet region. Therefore it is the commercial practice to broaden the sensitivity of the photoconductive materials over a Wider spectrum -by mixing small amounts of various dyes which are generally known as dye sensitizers with the-photoconductive materials. The dye sensitizers cause the photoconductive materials to be more sensitive, for example, to visible light. Many dye sensitizers can be used and are well known to those skilled in the electrostatic printing art. They are used in the practice of this invention to facilitate the use of the invention over a broader range of light spectrum. When dye sensitizers are used they are normally mixed with the photoconductive material in an amount of 0.1 to about 5 percent and preferably from about 0.1 to about 3 percent. Representative of the many suitable dye sensitizers are, for example, sensitizers of the phenolsulphonephthalein group, phthalein dyes, triphenylmethane dyes and cyanide dyes.

While certain representative embodiments and details have been shown for the purpose of illustrating the invention, it will be apparent to those skilled in this art that various changes and modifications may be made therein without departing from the spirit or scope of the invention.

What is claimed is:

1. An electrostatic printing material not requiring dark resting which comprises an electrically conductive support layer and a second layer, the said second layer comprising a polymeric binder containing a dispersion of a photoconductive material selected from thegroup consisting of selenium, zinc oxide, cadmium sulfide, cadmium telluride, anthracene and sulfur, where the said polymeric binder is a polymer having an intrinsic viscosity in chloroform at 30 C. of from about 0.2 to about 0.5 prepared by solution polymerization comprising from about 50 to about 65 percent by weight units derived from ethyl acrylate, from about 3 to about 10 percent by weight units derived from at least one compound selected from the group consisting of hydroxyethyl methacrylate and hydroxypropyl methacrylate, from about 30 to about 45 percent by weight units derived from styrene and up to about 10 percent by weight units derived from monochlorostyrene.

2. An electrostatic printing material according to claim 1 where the polymeric binder contains a dispersion of a photoconductive material and a dye sensitizer and the electrically conductive support layer has a specific conductivity of at least about 10- ohm- 3. The electrostatic printing material according to claim 2 where the solution polymerization is conducted at a temperature of from about 70 C. to about 120 C., with the solvent mixture comprising from about 50 to about 90 Weight percent aromatic hydrocarbons and correspondingly from about 50 to about weight percent aliphatic hydrocarbons and where the conversion of the monomers is at least about 97 percent.

4. An electrostatic printing material according to claim 1 where the photoconductive material is a particulate zinc oxide.

5. A method of preparing the electrostatic printing material of claim 1 which comprises coating an electrically conductive support with a solution of a polymeric binder containing a photoconductive material and drying the mixture of the polymeric binder and photoconductive material where the said polymeric binder is prepared by solution polymerizing a mixture of monomers which comprises from about 50 to about 65 percent by weight ethyl acrylate, from about 3 to about 10 percent by weight of at least one compound selected from the group consisting of hydroxyethyl methacrylate and hydroxypropyl methacrylate, from about to about percent by weight styrene and up to about 10 percent by weight units derived from monochlorostyrene.

6. A method according to claim 5 where the said mixture of monomers is polymerized in the presence of a free radical initiator and a solvent which will yield a polymer having an intrinsic viscosity of from about 0.2 to about 0.5 when the conversion of the mixture of monomers to the polymer is at least about 97 percent complete.

7. A method according to claim 6 where the solvent is a mixture comprising from about to about percent by weight aromatic hydrocarbons and from about 50 to about 10 percent by weight of compounds selected from the group consisting of aliphatic hydrocarbons, aliphatic alcohols and alcohol ethers.

8. A method according to claim 7 where the aromatic hydrocarbons are selected from the group consisting of xylene, benzene, and toluene, the aliphatic hydrocarbons are selected from the group consisting of the pentanes, the hexanes, the heptanes, and the octanes, the aliphatic alcohols are selected from the group consisting of ethyl alcohol, propyl alcohol, and butyl alcohol, and the alcohol ether is a mono methyl ether of ethylene glycol.

9. A method according to claim 6 where the solution of the polymeric binder contains a dispersion of a photoconductive material and a dye sensitizer and where the photoconductive material is selected from at least one of the group consisting of selenium, zinc oxide, cadmium sulfide, cadmium telluride, anthracene and sulfur.

References Cited UNITED STATES PATENTS 3,245,833 4/1966 Trevoy 117--201 3,251,687 5/1966 Fohl et a1 96-1 3,447,957 6/1969 Behringer 117-201 3,454,415 7/1969 Bonjour 11734 GEORGE F. LESMES, Primary Examiner M. B. WITTENBERG, Assistant Examiner US. Cl. X.R. 

